Many spacecraft, including satellites, include attitude control systems that are used to maneuver the spacecraft to a desired attitude. A typical attitude control system includes one or more control units, one or more sensors, and a plurality of reaction wheel assemblies (RWA). The sensors, which may include, for example, one or more sun sensors, earth sensors, and/or inertial sensors, determine actual spacecraft attitude and supplies signals representative of actual spacecraft attitude to the control unit. The control unit also receives attitude commands from either an on-board source or an external source, and determines whether an attitude adjustment needs to be made. If an attitude adjustment does indeed need to be made, the control unit supplies appropriate commands to the RWAs, which cause the rotational speed of the RWAs to either increase or decrease, thereby generating an appropriate torque that causes the spacecraft to move to the commanded attitude.
As is generally known, RWAs typically include a wheel mass, such as a flywheel, rotationally mounted within a housing that is coupled to the spacecraft structure, and a motor. The motor is typically implemented using a brushless DC motor. The speed of the motor is controlled via a motor control unit, which may be shared by one or more other RWAs or dedicated to a single RWA. In either case, the motor control unit receives commands supplied from the attitude control system control unit and, in response, controls the motor to either increase or decrease the rotational speed of the flywheel. The change in rotational speed of the flywheel results in a change in momentum. This change in momentum generates the torque that is supplied to the spacecraft. Although RWAs generally provide a safe, reliable, and cost effective way to generate spacecraft torques, presently known RWAs do suffer certain drawbacks. For example, presently known RWAs typically generate relatively low maximum torque magnitudes of, for example, about 1.5 Newton-meters (N-m) or less.
In some instances, it would be desirable to generate higher levels of torque from the RWA motor. To do so, the RWA could be implemented with multiple motors. This solution, however, could increase RWA cost and complexity. Alternatively, the current drivers that are used to supply current to the RWA motor could be redesigned to supply greater current magnitudes, or the bus voltage within the spacecraft could be increased. These latter alternatives may also be cost prohibitive, and therefore undesirable.
Hence, there is a need for the ability to generate relatively high levels of torque (e.g., greater than about 1.5 N-m) in an RWA from a single brushless DC motor, while utilizing readily available motor current drivers and/or without incurring the potential increased costs associated with changing spacecraft bus voltage magnitude. The present invention addresses at least this need.